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MODE OF SPREAD OF INFECTIVE DISEASES. 



Multiplicity 

 of the local 

 and seasonal 

 variations. 



mata, are not distributed over a wide area by means 

 of currents of air ; on the contrary, with the distance 

 from the patient and the progressive dilution by 

 pure air there is evidently such a rapid diminution 

 of the chances of infection that they nre no longer suffi- 

 cient for a natural spread of the disease ; on the other 

 hand, where the transport is by the air the chances of 

 infection only exist in the immediate neighbourhood of 

 the sources of infection, and when the infective agents 

 pass over greater distances they are usually carried by 

 means of man or by surrounding objects. 



The patient may therefore form the centre of a small 

 infectious area; from this new centres may then be 

 established in all possible directions, these centres having 

 their origin in infected patients, or in any other suitable 

 source of infection. The immediate neighbourhood of 

 such a centre is always specially dangerous ; whether, 

 however, a number of infections will occur in the same 

 house, or whether the danger will disappear there with- 

 out the occurrence of any new infection while infective 

 agents may possibly have been carried to other streets or 

 places, and give rise to fresh areas of infection, depends 

 in every individual case on a great variety of circum- 

 stances on the number and resisting power of the sources 

 of infection, on the number of modes of transport, on 

 the individual susceptibility and may vary to a great 

 extent. We must, therefore, after what has been said 

 in the foregoing chapters as to the multiplicity of all the 

 factors which come into play in infection, look on the 

 whole essence of distribution as something so dherse 

 and varying, and we must realise that the paths are so 

 tortuous and the results so unexpected, that we need not 

 be surprised either by a persistent localisation, or by an 

 apparently causeless leap of the disease over a consider- 

 able distance. 



It is only where a community is overtaken by a new 

 and very contagious disease, where total ignorance of 

 the danger of infection renders every mode of transmis- 

 sion easier, and where every individual presents a predis- 

 posed body not yet protected by recovery from similar 



